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Plastics, organic polymeric materials (those consisting of giant organic molecules) that are plastic—that is, they can be formed into desiredshapes through extrusion, moulding, casting, or spinning. The molecules can be either natural—including cellulose, wax, and natural rubber— or synthetic—including polyetheneand nylon. The starting materials are resins in the form of pellets, powders, or solutions; from these are formed the finished plastics.

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Plastics are characterized by high strength-to-density ratios, excellent thermal andelectrical insulation properties, and goodresistance to acids, alkalis, and solvents.The giant molecules of which they consistmay be linear, branched, or cross-linked,depending on the plastic. Linear andbranched molecules are thermoplastic(soften when heated), whereas cross-linkedmolecules are thermosetting (harden whenheated).

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HISTORYThe development of plastics began about 1860, afterPhelan and Collander, a United States firmmanufacturing billiard and pool balls, offered a prize of$10,000 for a satisfactory substitute for natural ivory.One of those who tried to win this prize was a USinventor, John Wesley Hyatt. Hyatt developed a methodof pressure-working pyroxylin, a cellulose nitrate of lownitration that had been plasticized with camphor and aminimum of alcohol solvent.

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In the early 1940s, Lou Glaser, a California entrepreneur, founded aninjection molding company. Precision Specialties performed contractwork for other manufactures. In the early 1950s Gowland and Gowlanddesigned the famous 1/16 scale “Highway Pioneers” line of 34 cars,which were the first mass-produced plastic automotive kits. Glasermarketed these for 69 cents through Woolworth Dime Stores and theysold well. Glaser realized that Revell should sell children’s toys,specifically plastic model kits.

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One of those who tried to win this prize was a USinventor, John Wesley Hyatt. Hyatt developed amethod of pressure-working pyroxylin, a cellulosenitrate of low nitration that had been plasticizedwith camphor and a minimum of alcohol solvent.Although Hyatt did not win the prize, his product,patented under the trademark Celluloid, was usedin the manufacture of objects ranging from dentalplates to mens collars. Despite its flammabilityand liability to deterioration when exposed tolight, Celluloid achieved a notable commercialsuccess.

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Other plastics were introduced graduallyover the next few decades. Among themwere the first totally synthetic plastics: thefamily of phenol-formaldehyde resinsdeveloped by the Belgian-American chemistLeo Hendrik Baekeland about 1906 andsold under the trademark Bakelite. Otherplastics introduced during this periodinclude modified natural polymers such asrayon, made from cellulose products.

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A Breakthrough in Plastics ChemistryIn 1920 an event occurred that set the stagefor the future rapid development of plasticmaterials. The German chemist HermannStaudinger conjectured that plastics weretruly giant molecules. His subsequent effortsto prove this claim initiated an outburst ofscientific investigation that resulted in majorbreakthroughs in the chemistry of plastics.

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Throughout the 1920s and 1930s largenumbers of new products were introduced,including cellulose ethanoate (originallycalled cellulose acetate), used in mouldingresins and fibres; PVC—polyvinyl chloride,or polychloroethene—used in plastic pipe,vinyl coatings, and wire insulation; urea-formaldehyde resins, used in tableware andelectrical applications; and acrylic resin,developed as a binder for laminated glass.

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One of the most familiar plastics developed in thisperiod is polymerized methyl methacrylate, whichis marketed in Britain as Perspex and in theUnited States as Lucite and Plexiglas. Thismaterial has excellent optical properties and issuitable for spectacles and camera lenses and forstreet and advertising illumination. Polystyreneresins, also first produced commercially about1937, are characterized by high resistance tochemical and mechanical alteration at lowtemperatures and by very low absorption ofwater.

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The polystyrenes are especially suitable forradio-frequency insulation and for accessoriesused in low temperatures, as in refrigerationinstallations and in aeroplanes designed forhigh-altitude flight. PTFE(polytetrafluoroethene), first made in 1938,was eventually produced commercially asTeflon in 1950; it is the coating used on non-stick cooking utensils. Another keydevelopment during the 1930s was thesynthesis of nylon, the first high-performanceengineering plastic.

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B .World WarIIDuring World War II, both the Allies and the Axispowers were faced with severe shortages ofnatural raw materials. The plastics industryproved to be a rich source of acceptablesubstitutes. Germany, for example, which wassoon cut off from sources of natural latex, initiateda major programme that led to the developmentof a practical synthetic rubber.

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Japans entry into the war eliminated most of theUnited States Far Eastern sources of naturalrubber, silk, and many metals. The US responsewas to accelerate the development andproduction of plastics. Nylon became a majorsouce of textile fibres, polyesters were used infabricating armour and other war materials, andvarious types of synthetic rubber were producedin quantity.

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C . The Post-War BoomThe scientific and technological momentum in theplastics industry carried over into the post-waryears. Of particular interest were the advances insuch engineering plastics as polycarbonates,acetals, and polyamides; other synthetics wereused in place of metal in machinery, safetyhelmets, high-temperature devices and manyother products used in environmentallydemanding settings.

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In1953 the German chemist Karl Zieglerdeveloped polyethene (originally calledpolyethylene), and in 1954 the Italian chemistGiulio Natta developed polypropene (originallycalled polypropylene)—two of todays mostimportant plastics. A decade later, these two menshared the 1963 Nobel Prize for Chemistry fortheir studies of polymers.

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III . KINDS OF PLASTICSThree of the ways in which plastics can becategorized are by the polymerization processthat forms them, by their processibility, and bytheir chemical nature. A. PolymerizationThe two basic polymerization processes forproducing resins are condensation and additionreactions. Condensation produces a variety oflengths of the chain of monomers (repeatingunits), whereas addition reactions produce onlyspecific lengths.

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B. ProcessibilityThe processibility of a plastic depends onwhether it is thermoplastic or thermosetting.Thermoplastics, which are made up of linear orbranched polymers, are fusible: they soften whenheated and harden when cooled. This is also trueof thermosets that are lightly cross-linked. Mostthermosets, however, harden when heated. Thisfinal cross-linking, which fixes the truethermosets, takes place after the plastic hasalready been formed.

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C .Chemical NatureThe chemical nature of a plastic is defined by themonomer that makes up the chain of the polymer. Forexample, polyolefins are made up of monomer units ofolefins, which are open-chain hydrocarbons with at leastone double bond. Polyethene is a polyolefin; itsmonomer unit is ethene (formerly called ethylene). Othercategories are acrylics (such aspolymethylmethacrylate), styrenes (such aspolystyrene), vinyl halides (such as polyvinyl chloride),polyesters, polyurethanes, polyamides (such as nylons),polyethers, acetals, phenolics, cellulosics, and aminoresins.

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IV MANUFACTUREThe manufacture of plastic and plasticproducts involves procuring the rawmaterials, synthesizing the basic polymer,compounding the polymer into a materialuseful for fabrication, and moulding orshaping the plastic into its final form.

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A Raw MaterialsOriginally, most plastics were made from resins derivedfrom vegetable matter, such as cellulose (from cotton),furfural (from oat hulls), oils (from seeds), starchderivatives, or coal. Casein (from milk) was among thenonvegetable materials used. Although the production ofnylon was originally based on coal, air, and water, andnylon 11 is still based on oil from castor beans, mostplastics today are derived from petrochemicals. Theseoil-based raw materials are relatively widely availableand inexpensive. However, because the world supply ofoil is limited, other sources of raw materials, such ascoal gasification, are being explored.

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B Synthesizing the PolymerThe first stage in manufacturing plastic is polymerization.The two basic polymerization processes of condensationand addition reactions may be carried out in variousways. In bulk polymerization, the pure monomer alone ispolymerized, generally either in the gaseous or liquidphase, although a few solid-state polymerizations arealso used. In solution polymerization, an emulsion isformed and then coagulated. In interfacialpolymerization, the monomers are dissolved in twoimmiscible liquids, and the polymerization occurs at theinterface of the two liquids.

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C AdditivesChemical additives are often used in plastics to produce somedesired characteristic. For instance, antioxidants protect apolymer from chemical degradation by oxygen or ozone; similarly,ultraviolet stabilizers protect against weathering. Plasticizers makea polymer more flexible, lubricants reduce problems with friction,and pigments add colour. Among other additives are flameretardants and antistatics.Many plastics are manufactured as composites. This involves asystem where reinforcing material (usually fibres made of glass orcarbon) is added to a plastic resin matrix. Composites havestrength and stability comparable to that of metals but generallywith less weight. Plastic foams, which are composites of plasticand gas, offer bulk with low weight.

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D Shaping and FinishingThe techniques used for shaping and finishing plastics depend on three factors:time, temperature, and flow (also known as deformation). Many of theprocesses are cyclic in nature, although some fall into the categories ofcontinuous or semicontinuous operation.One of the most widely used operations is that of extrusion. An extruder is adevice that pumps a plastic through a desired die or shape. Extrusion products,such as pipes, have a regularly shaped cross section. The extruder itself alsoserves as the means to carry out other operations, such as blow moulding andinjection moulding. In extrusion blow moulding, the extruder fills the mould witha tube, which is then cut off and clamped to form a hollow shape called aparison. The hot, molten parison is then blown like a balloon and forced againstthe walls of the mould to form the desired shape. In injection moulding, one ormore extruders are used with reciprocating screws that move forwards to injectthe melt and then retract to take on new molten material to continue theprocess. In injection blow moulding, which is used in making bottles forcarbonated drinks, the parison is first injection moulded and then reheated andblown.

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In compression moulding, pressure forces the plasticinto a given shape. Another process, transfer moulding,is a hybrid of injection and compression moulding: themolten plastic is forced by a ram into a mould. Otherfinishing processes include calendering, in which plasticsheets are formed, and sheet forming, in which theplastic sheets are formed into a desired shape. Someplastics, particularly those with very high temperatureresistance, require special fabrication procedures. Forexample, polytetrafluoroethene has such a high meltviscosity that it is first pressed into shape and thensintered—exposed to extremely high temperatures thatbond it into a cohesive mass without melting it. Somepolyamides are produced by a similar process.

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V USESPlastics have an ever-widening range of uses in both the industrial and consumersectors.A PackagingThe packaging industry is a leading user of plastics. Much LDPE (low-densitypolyethene) is marketed in rolls of cling film. High-density polyethene (HPDE) is used forsome thicker plastic films, such as those used for plastic waste bags and containers.Other packaging plastics include polypropene, polystyrene, PVC, and polyvinylidenechloride. Polyvinylidene chloride is used primarily for its barrier properties, which cankeep gases such as oxygen from passing into or out of a package. Similarly,polypropene is an effective barrier against water vapour. Polypropene is also often usedin housewares and as a fibre for carpeting and rope.

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Construction The building industry is a major consumer of plastics, including many of the packaging plastics mentioned above. HDPE is used for pipes, as is PVC; PVC is also used in sheets for building materials and similar items. Many plastics are used to insulate cables and wires, and polystyrene in the form of foam serves as insulation for walls, roofs, and other areas. Other plastic products are roofing, door and window frames, mouldings, and hardware.

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C Other UsesMany other industries, especially motor manufacturing, also depend on plastics. Toughengineering plastics are found in vehicle components like air-intake manifolds, fuel lines,emission canisters, fuel pumps, and electronic devices. Plastics are also used for interiorpanelling, seats, and trim. Car bodies can be made of fibreglass-reinforcedplastic.Among the other uses of plastic are housings for business machines, electronicdevices, small appliances, and tools. Consumer goods range from sports equipment toluggage and toys.VI HEALTH AND ENVIRONMENTAL HAZARDSBecause plastics are relatively inert, the final products do not normally present healthhazards to the maker or user. However, some monomers used in the manufacture ofplastics have been shown to cause cancer.

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Similarly, benzene, which is an important raw material for thesynthesis of nylon, is a carcinogen. The problems involved in themanufacture of plastics parallel those of the chemical industry ingeneral.Most synthetic plastics are not environmentallydegradable; unlike wood, paper, natural fibres, or even metal andglass, they do not rot or otherwise break down over time. (Somedegradable plastics have been developed, but none has provedcompatible with the conditions required for most waste landfills.)Thus, there is an environmental problem associated with thedisposal of plastics. Recycling has emerged as the most practicalmethod to deal with this problem, especially with products such asthe polyethene terephlalate bottles used for carbonated drinks,where the process of recycling is fairly straightforward. Morecomplex solutions are being developed for handling thecommingled plastic scrap that constitutes a highly visible, albeitrelatively small, part of the problem of solid waste disposal.

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Types of plasticsCELLULOIDCelluloid, originally the trade name and now thecommon name of a synthetic plastic made bymixing cellulose nitrate, or pyroxylin, withpigments and fillers in a solution of camphor inalcohol. When heated, the substance is pliable orplastic and can be moulded into a variety ofshapes. Upon drying and cooling, the materialbecomes hard.

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In the United States celluloid was invented by JohnHyatt, who was trying to win a $10,000 award for findinga substitute for ivory in making billiard balls. Hyatt failedto win the prize, but he received a patent for hisdiscovery in 1870. The patent was disputed by theBritish inventor of Xylonite, a similar product.Celluloid istransparent and colourless and in paste form can becoloured or rolled or moulded into specific shapes.Some of its advantages are that it is inexpensive anddurable, takes a high polish, does not warp or discolour,and is not affected by moisture. It is, however, highlyflammable, and although modifications in manufacturehave reduced the dangers of fire, it has been largelysuperseded by other materials. Celluloid is used inmaking combs, brushes, and buttons.

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CELLULOSECellulose (Latin cellula,”little cell”), apolysaccharide (long chain polymer made up ofsugars), the chief constituent of the cell wall of allplant cells. In plants, cellulose is normallycombined with woody, fatty, or gummysubstances. With some exceptions amonginsects, true cellulose is not found in animaltissues. Micro-organisms in the digestive tracts ofherbivorous animals break down the celluloseinto products that can be absorbed. Cellulose isinsoluble in all ordinary solvents and may bereadily separated from the other constituents ofplants.

36.
Depending on its concentration, sulphuric acid acts on cellulose toproduce glucose, soluble starch, or amyloid; the last is a form ofstarch used for the coating of parchment paper. When cellulose istreated with an alkali and then exposed to the fumes of carbondisulphide, the solution yields films and threads. Rayon andcellophane are cellulose regenerated from such solutions.Cellulose acetates are spun into fine filaments for the manufactureof some fabrics and are also used for photographic safety film, asa substitute for glass, for the manufacture of safety glass, and asa moulding material. Cellulose ethers are used in paper sizings,adhesives, soaps, and synthetic resins.With a mixture of nitricacid and sulphuric acid, cellulose forms a series of flammable andexplosive compounds known as cellulose nitrates, ornitrocelluloses. Pyroxylin, also called collodion cotton, is a nitrateused in various lacquers and plastics; another, collodion, is usedin medicine, photography, and the manufacture of artificial leatherand some lacquers. A third nitrate, guncotton, is a propellantexplosive used in cartridges.

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FibreI INTRODUCTIONFibre, fine hair-like structure of animal, vegetable, mineral, or synthetic origin. Fibresaverage less than 0.05 cm (0.02 in) in diameter. Used for textiles and for many otherproducts, fibres are classified according to their origin, their chemical structure, or both.II ANIMAL FIBRESChemically, all animal fibres are complex proteins. They are resistant to most organicacids and, under proper circumstances, to certain strong mineral acids such as sulphuricacid (H2 SO4). But protein fibres may be damaged even by mild alkalis and may betotally dissolved by such strong alkalis as sodium hydroxide (NaOH). These fibres arealso subject to damage by chlorine-based bleaches. Liquid hypochlorite bleach shouldnever be used on wool or silk. If used undiluted, it will damage fibres or even completelydissolve them.

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The principal component of silk is the protein fibroin. Silk is extruded in continuousfilaments from the abdomens of various insects and spiders. It is the only naturalfilament—a term used to describe a fibre of indefinite length—commonly reaching alength of more than 1,000 m (3,300 ft). Several silk filament fibres are gathered toproduce a filament yarn. Silk, however, is often produced and utilized in short or stapleform to manufacture spun yarns. True silk as used in textiles is produced by only oneinsect, the silkworm.The principal component of hair, wool, and fur, the protective skin hairs of mammals, isthe protein keratin. Fibres of hair and wool are not continuous and must be spun intothread or yarn if they are to be woven or knitted into textiles, or they must be felted.The chief hair fibre used to produce textile fabrics is sheeps wool. Individual hairs maybe as long as 90 cm (36 in) but are usually no more than 40 cm (16 in). In wild sheep thewool is a short, soft underlayer protected by longer, coarser hairs; in domesticatedsheep bred for their fleece, the length of the wool is greatly increased. All hair fibreshave a coat of overlapping scales. The size and shape of the scales is unique to eachspecies. In many kinds of sheep wool the scales are quite pronounced. Wool fibres thatare not smooth but naturally crimped produce air-trapping yarns used for insulatingmaterials.

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Other animals used as sources of hair fibre for textiles include the llama, alpaca, vicuña,Angora goat and rabbit, Kashmir goat, and camel. The vicuña, native to the Andes, isnow considered an endangered species because of overhunting. Fur fibres from animalssuch as mink and beaver are sometimes blended with other hairs to spin luxury yarnsbut the pelts are more often used. Horsehair and cows hair are used for felts and aresometimes spun as yarn, particularly for upholstery and other applications wheredurability is important. Even human hair has been spun into yarn and used for textiles.III VEGETABLE FIBRESVegetable fibres are predominantly cellulose, which, unlike the protein of animal fibres,resists alkalis. Vegetable fibres resist most organic acids but are destroyed by strongmineral acids. Improper use of most bleaches can weaken or destroy these fibres.The chief vegetable fibres are structurally of four kinds: seed fibres, the soft hairs thatsurround the seeds of certain plants; bast fibres, the tough fibres that grow between thebark and stem of many dicotyledonous plants (see Dicots); vascular fibres, the toughfibres found in the leaves and stems of monocotyledons (see Monocots); and entirestems of grasses.

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Certain other structural types have limited utility. These include strips of leaf skins, suchas raffia; fibres of the fruit case, such as coir, and palm fibres. Only two seed-hair fibres,cotton and kapok, have commercial importance. Cotton, the most adaptable and mostwidely used of all fibres, is the only seed fibre with textile utility. Kapok cannot be spunbut is used as upholstery stuffing. Because it is hollow, kapok is buoyant. It has beenused for flotation devices such as life preservers, but today it has largely been replacedby other materials.The numerous varieties of bast fibres are used for purposes rangingfrom weaving fine textiles to manufacturing cordage. Linen cloth is made from flax, andcoarser cloths and rope and twine are produced from hemp, jute, ramie, andsunn.Vascular fibres are used almost entirely for cordage making. They include agave(sisal), henequen, manila hemp, yucca, and a number of others. The vascular fibres ofthe pineapple have been used in the production of textiles.Entire stems of some grassesand straws are woven as fibres for hats and matting. Among such fibres isesparto.Vegetable fibres have extensive application in making paper. Cotton and flaxform the basis for fine rag papers, and grasses, hemp, jute, and manila are often used inmaking wrapping papers and other coarse papers. Newsprint and kraft papers areproduced from wood fibre after chemical treatment. Wood fibre and bagasse, the fibre ofsugar cane, are made into building board by a process analogous to papermaking.

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IV MINERAL FIBRESOnly one fibre of inorganic (mineral) origin is used to any great extent in conventionalfabrics: glass fibre, made by drawing or blowing molten glass into threads. Fibres ofasbestos, formerly used for insulation and fireproofing, have been found to becarcinogenic. Thin metal wire is sometimes used for the production of gauze and iswoven with organic fibres to give special patterns. Most so-called metal thread, however,actually consists of thin strips of metal foil similar to tinsel. To impart strength, metallicfoils are often sandwiched between layers of plastic film. Other metallic yarns consist ofa cotton core wound with a thin metal strip or thread that has been coated with a viscoussubstance and dipped in metal powder. The insulation material called rock wool is afibrous substance made from steel-mill slag, limestone, or siliceous rock.V SYNTHETIC FIBRESSynthetic fibres derived from natural cellulose were first developed at the end of the 19thcentury and came to be known as rayons. Rayons are called regenerated fibres sincethe cellulose is not synthesized. Natural cellulose in forms lacking textile utility, such aswood fibre, is chemically changed into compounds that can be liquefied. These liquidsare extruded as filaments into an environment that converts them back into pure, solidcellulose, from which rayons are made.

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Acetates and triacetates, which are true synthetics, were developed shortly after rayons;they are plastics derived from cellulose in a process similar to that used for rayon,except that the cellulose is chemically altered to form esters.Most synthetic fibres are now made from petrochemicals and are giant polymersresembling plastics in structure. The first commercially successful plastic fibre, nylon,dates from 1938. Since then many synthetics, including acrylics, aramids, olefins, andpolyesters, have been developed. With synthetics, as with rayons and acetates, fibre-forming liquids are extruded as filaments into an environment that causes them tosolidify. They are then treated to yield such qualities as heat and moisture resistance,ease of dyeing, and stretchability.Synthetic fibres have also been developed for high-performance industrial applications,such as bulletproof fabrics, insulation materials, and aircraft fuselages and wings; foruse in space exploration; and for sports equipment of all types. In order to obtainqualities such as great strength and high heat resistance, synthetic fibres may becombined with fibres of carbon, boron, silicon, or other substances.

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Vinyl ChlorideVinyl Chloride, gas with the chemical composition CHClCH2, which when polymerizedproduces polyvinyl chloride, or vinyl plastic. The gas, formed by reacting ethene orethyne with hydrochloric acid, was once also used as a propellant in aerosols but wasfound to be a carcinogen.PolymerI INTRODUCTIONPolymer, substance consisting of large molecules that are made of many small,repeating units called monomers. The number of repeating units in one large molecule iscalled the degree of polymerization. Materials with a very high degree of polymerizationare called high polymers. Polymers consisting of only one kind of repeating unit arecalled homopolymers. Copolymers are formed from several different repeatingunits.Most of the organic substances found in living matter, such as protein, wood, chitin,rubber, and resins, are polymers. Many synthetic materials, such as plastics, fibres (seeNylon; Rayon), adhesives, glass, and porcelain, are also largely polymeric substances.

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Polymers can be subdivided into three or four structural groups.The molecules in linear polymers consist of long chains ofmonomers joined by bonds, like beads on a necklace. Typicalexamples are polyethene, polyethenol—formerly called polyvinylalcohol—and PVC (polyvinyl chloride, or polychloroethene).Branched polymers have side chains that are attached to thechain molecule itself. Branching can be caused by impurities or bythe presence of monomers that have several reactive groups.Chain polymers composed of monomers with side groups that arepart of the monomers, such as polyphenylethene (polystyrene) orpolypropene, are not considered branched polymers.In cross-linked polymers, two or more chains are joined together by sidechains.With a small degree of cross-linking, a loose network is obtainedthat is essentially two dimensional. High degrees of cross-linkingresult in a tight three-dimensional structure. Cross-linking isusually caused by chemical reactions.

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III SYNTHESISTwo general methods exist for forming large molecules from smallmonomers: addition polymerization and condensationpolymerization. In the chemical process called additionpolymerization, monomers join together without the loss of atomsfrom the molecules. Some examples of addition polymers arepolyethene, polypropene, polyphenylethene, polyethenylethanoate, and polytetrafluoroethylene (Teflon).In condensationpolymerization, monomers join together with the simultaneouselimination of atoms or groups of atoms. Typical condensationpolymers are polyamides, polyesters, and certain polyurethanes.In 1983 a new method of addition polymerization called grouptransfer polymerization was announced. An activating group withinthe molecule initiating the process transfers to the end of thegrowing polymer chain as individual monomers insert themselvesin the group. The method has been used for acrylic plastics; itshould prove applicable to other plastics as well.

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NylonNylon, a synthetic polymer widely used for textile fibres,characterized by great strength, toughness, and elasticity, andprocessed also in the form of bristles and moulded articles. Nylonwas developed in the 1930s by scientists of E. I. du Pont deNemours and Company, Inc., headed by the American chemistWallace Hume Carothers. It is usually made by polymerizingadipic acid and hexamethylenediamine, an amine derivative(seePolymer: Synthesis). Adipic acid is derived from cyclohexaneby an oxidation reaction that opens up the ring of carbon atoms;hexamethylenediamine is made by treating adipic acidcatalytically with ammonia and hydrogenating the product(seeHydrogenation). Nylon is insoluble in water and in ordinaryorganic solvents; it dissolves in phenol, cresol, and formic acid,and melts at 263° C (505° F).

50.
In making textile fibres, small chips of the nylon polymer, which isobtained as a tough, ivorylike material, are melted and forcedthrough holes in a metal disc called a spinneret. The filaments arecongealed by a blast of air and are then drawn to about four timestheir original lengths. The diameter of the filaments is controlledby changing the rate at which the molten nylon is pumped into thespinneret and the rate at which the filaments are drawn away.Filaments much finer than those of ordinary textile fibres can bemade from nylon. Nylon fibres can have the appearance andlustre of silk or can be made to resemble natural fibres such ascotton; their tensile strength is higher than that of wool, silk, rayon,or cotton. Dyes are applied either to the molten mass of nylon orto the yarn or finished fabric. Acetate rayon dyes are usually usedfor nylon.Nylons made from other acids and amines resemble, in general,the nylon described above.

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Nylon is used in the manufacture of fabrics for such articles ashosiery, sleepwear, underwear, blouses, shirts, and raincoats.Nylon fabrics are water-resistant; they dry quickly when launderedand usually require little or no ironing. Nylon fibres are also usedfor parachutes, insect screening, medical sutures, strings fortennis rackets, brush bristles, rope, and fishing nets and line.Moulded nylon is used for insulating material, combs, kitchenutensils, and machinery parts.

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InsulationI INTRODUCTIONInsulation, any material that is a poor conductor of heat orelectricity, and that is used to suppress the flow of heat orelectricity.II ELECTRIC INSULATIONThe perfect insulator for electrical applications would be a material that isabsolutely non-conducting; such a material does not exist. The materials usedas insulators, although they do conduct some electricity, have a resistance tothe flow of electric current as much as 2.5 × 1024 greater than that of goodelectrical conductors such as silver and copper. Materials that are goodconductors have a large number of free electrons (electrons not tightly bound toatoms) available to carry the current; good insulators have few such electrons.Some materials such as silicon and germanium, which have a limited number offree electrons, are semiconductors and form the basic material of transistors.

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In ordinary electrical wiring, plastics are commonly used asinsulating sheathing for the wire itself. Very fine wire, such as thatused for the windings of coils and transformers, may be insulatedwith a thin coat of enamel. The internal insulation of electricalequipment may be made of mica or glass fibres with a plasticbinder. Electronic equipment and transformers may also use aspecial electrical grade of paper. High-voltage power lines areinsulated with units made of porcelain or some other ceramic, orof glass.The specific choice of an insulation material is usuallydetermined by its application. Polyethylene and polystyrene areused in high-frequency applications, and mylar is used forelectrical capacitors. Insulators must also be selected according tothe maximum temperature they will encounter. Teflon is used inthe high-temperature range of 175° to 230° C (350° to 450° F).Adverse mechanical or chemical conditions may call for othermaterials. Nylon has excellent abrasion resistance, and neoprene,silicone rubber, epoxy polyesters, and polyurethanes can provideprotection against chemicals and moisture.

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III THERMAL INSULATIONThermal insulating materials are used to reduce the flow of heatbetween hot and cold regions. The sheathing often placed aroundsteam and hot-water pipes, for instance, reduces heat loss to thesurroundings, and insulation placed in the walls of a refrigeratorreduces heat flow into the unit and permits it to stay cold.Thermal insulation may have to fulfil one or more of threefunctions: to reduce thermal conduction in the material, in whichheat is transferred by electrons; to reduce thermal convectioncurrents, which can be set up in air- or liquid-filled spaces; and toreduce radiation heat transfer, in which thermal energy istransported by electromagnetic waves. Conduction andconvection are suppressed in a vacuum, in which radiation is theonly method of transferring heat. If the surfaces are made highlyreflective, radiation can also be reduced.

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Thus, thin aluminium foil can be used in building walls, andreflecting metal on roofs minimizes the heating effect of the Sun.Thermos bottles or Dewar flasks (see Cryogenics) retain orexclude heat becaues they have double walls that have reflectivesilver or aluminium coatings and are separated by a vacuum. SeeAlso Heat Transfer.Air offers about 15,000 times as muchresistance to heat flow as a good thermal conductor such as silverdoes, and about 30 times as much as glass. Typical insulatingmaterials, therefore, are usually made of non-metallic materialsand are filled with small air pockets. They include magnesiumcarbonate, cork, felt, cotton batting, rock or glass wool, anddiatomaceous earth. Asbestos was once widely used for insulation, but it has beenfound to be a health hazard and has, therefore, been banned innew construction in many countries.In building materials, airpockets provide additional insulation in hollow glass bricks,double-glazed windows (consisting of two or three sealed glasspanes with a thin air space between them), and partially hollow

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Insulating properties become poorer if the air space becomeslarge enough to allow thermal convection, or if moisture seeps inand acts as a conductor. The insulating property of dry clothing,for example, is the result of air trapped between the fibres; thisability to insulate can be significantly reduced by moisture.Home-heating and air-conditioning costs can be reduced byproper building insulation. In cold climates about 8 cm (about 3 in)of wall insulation and about 15 to 23 cm (about 6 to 9 in) of ceilinginsulationarerecommended.Superinsulation has recently beendeveloped, primarily for use in space, where protection is neededagainst external temperatures near absolute zero. Superinsulationfabric consists of multiple sheets of aluminized Mylar, each about0.005 cm (0.002 in) thick, separated by thin spacers, with about20 to 40 layers per cm (50 to 100 layers per in).